1. Field of the Invention
The present invention relates to a variable gain amplifier, and more particularly, to a wideband variable gain amplifier with high linearity that operates in a switch mode.
2. Background of the Invention
A variable gain amplifier refers to a device which functions to maintain a desired output signal level by controlling its gain. The variable gain amplifier is usually used in radio frequency-receiving devices in which a signal received through an antenna has a high dynamic range. In order for a signal of a constant level to be supplied to a base band terminal of the received signal, the variable gain amplifier must also have a high dynamic range.
Many researches have been actively made so far on a variable gain amplifier, and hence various variable gain amplifiers have been proposed. Researches are in progress to increase the dynamic range of the variable gain amplifier and improve a linearity of a signal.
One example of prior arts attempted to improve a linearity of a variable gain amplifier includes U.S. Pat. No. 5,949,286.
The prior art amplifier includes a differential amplifier transistor pair for amplifying an input signal and a diode pair connected between the emitters of the transistor pair to provide its gain control using emitter degeneration.
In other words, the amount of current applied to a diode pair can be controlled by connecting a transistor circuit to the diode pair and varying a control voltage applied to the transistor. Therefore, an impedance value of the diode pair is controlled by the control voltage, so that a gain control of the variable gain amplifier can be accomplished.
The variable gain amplifier disclosed in U.S. Pat. No. 5,949,286, however, has the diodes connected to its input terminal for performing a predistortion function, thereby improving non-linear characteristics of the diode pair connected between the emitters of the transistor pair. Furthermore, a dummy amplifier is connected to the differential amplifier transistor pair to cancel the capacitive effects generated by the diode pair in order to increase the gain control range of the amplifier at higher frequencies.
For the variable gain amplifier disclosed in U.S. Pat. No. 5,949,286, however, since it employs the dummy amplifier to improve its linearity, there has been a problem in that additional current and chip area are needed, and in that there is a limit in improving linear-characteristics using an active element.